1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus which uses laser beams.
2. Description of the Related Art
An image forming apparatus such as a laser printer, a digital copying machine, or a facsimile machine includes: a photosensitive element of which the surface has photosensitivity as a scanning surface; a light source that emits laser beams; a polygon mirror that deflects the laser beams emitted from the light source; a scanning optical system that leads the laser beams deflected by the polygon mirror to a photosensitive element, and the like.
A light spot positioned on the photosensitive element moves in the axial direction of the photosensitive element in accordance with the rotation of the polygon mirror, thereby performing scanning corresponding to one line. Then, when the scanning corresponding to one line is completed, the photosensitive element rotates so as to start the next scanning.
Since the scanning optical system is configured by optical elements such as a lens, a glass plate, and a mirror, the light use efficiency (reflectance or transmittance) differs in accordance with the incidence angle of light. In addition, the thickness of the lens differs in accordance with the incident position of light.
Since the laser beams deflected by the polygon mirror are incident to the scanning optical system with the incidence angle corresponding to a defection angle in the polygon mirror, and the incident position in the scanning optical system differs depending on the irradiation position in the photosensitive element, the intensity of the laser beams on the photosensitive element is not uniform and differs in accordance with the irradiation position.
The variation in the intensity of the laser beams according to an irradiation position is called “shading characteristics” and is one of factors that degrade the image quality by generating a density variation in an image (also referred to as an “output image”) output from the image forming apparatus. Thus, various methods for correcting the shading characteristics have been proposed (for example, see Japanese Patent Application Laid-Open Nos. 2007-135100 and 2009-262344).
In addition, an image forming apparatus that controls the amount of exposure in accordance with a variation in the sensitivity of a photosensitive element is disclosed in Japanese Patent Application Laid-Open No. 2008-065270.
In a case where the photosensitive element is eccentric or has a cross-section that is not a perfect circle, when the photosensitive element rotates, a gap between the photosensitive element and a developing roller changes. Such a change in the gap causes a variation in a developing process, whereby an unnecessary density variation occurs in the output image.
Recently, there has been a strong request for improving the image quality, but it is difficult to suppress the density variation in an output image, which is caused by the eccentricity of the photosensitive element or a form error, to the requested level by using the conventional method.